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# 作者(Author)：
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# 来源(Source)：

#include<bits/stdc++.h>
#define ios ios_base::sync_with_stdio(0),cin.tie(0),cout.tie(0)
#define debug  freopen("in.txt","r",stdin),freopen("out.txt","w",stdout);
#define fs first
#define sc second
#define pb push_back
using namespace std;
typedef long long ll;
typedef pair<int,int> pii;
const int maxn = 1e6+10;
using namespace std;
 
ll a,b,c,k;
int main(){
    // debug;
    ios;
 
    cin>>a>>b>>c>>k;
    ll ans;
    if(k <= a) ans = k;
    else if(k <= a+b) ans = a;
    else ans = a - (k - a - b);
    cout<<ans<<'\n';
    return 0;
}
